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Performance comparison of mono-polar and bi-polar configurations of alkaline electrolysis stack through 3-D modelling and experimental fabrication
Journal Pre-proof Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication.
Y. Sanath K. De Silva, Peter Hugh Middleton, Mohan Kolhe PII:
S0960-1481(19)31959-7
DOI:
https://doi.org/10.1016/j.renene.2019.12.087
Reference:
RENE 12796
To appear in:
Renewable Energy
Received Date:
15 June 2019
Accepted Date:
19 December 2019
Please cite this article as: Y. Sanath K. De Silva, Peter Hugh Middleton, Mohan Kolhe, Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication., Renewable Energy (2019), https://doi.org/10. 1016/j.renene.2019.12.087
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof Title: Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication.
Author names and affiliations: Y. Sanath K. De Silvaa, Peter Hugh Middletonb and Mohan Kolheb a,bFaculty
of Engineering and Science, University of Agder (UiA), PO Box 422, NO 4604, Kristiansand, Norway. [email protected],[email protected]
and [email protected]
Corresponding author: Y. Sanath K. De Silva, Email: [email protected] Present Address: Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, University of Ruhuna, Hapugala, Galle.
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Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication. Y. Sanath K. De Silva, Peter Hugh Middleton, Mohan Lal Kolhe. Faculty of Engineering and Science, University of Agder, PO Box 422, NO 4604, Kristiansand, Norway. E-mails: [email protected], [email protected], [email protected] Abstract. Generation of hydrogen using electrolysis process with integrated renewable energy sources is highly important especially in environmental aspects. In this paper, we demonstrate that the enhancement of electrolysis performance of alkaline electrolysis stacks by diminishing the distance between electrodes, while changing the properties of the Membrane Electrode Assembly (MEA). Prior to that, the performances of mono-polar and bi-polar configurations of alkaline electrolysis stack are compared through 3-D modelling and experimental fabrication. At first, two different single cell alkaline electrolysers are designed using SolidWorks as a design software and the designed cell has been fabricated using an in-house 3D printer, to avoid post machining processes. Thereafter, the best performing cell is selected by considering the performance of both designs through different experiments. Finally, the performance of the selected cell is enhanced by changing the distance between electrodes and properties of MEA. Thus, the best performing cell has been selected for the fabrication process of mono-polar and bi-polar electrolysis stack. At last, both mono-polar and bi-polar configurations of alkaline electrolysis stacks are designed and implemented to compare the electrolysis performance of both configurations by maintaining minimum electrode distance. The results imply that the electrolysis performance of the cell can be enhanced by reducing the distance between electrodes, and the designed bi-polar stack has a better performance in terms of the efficiency, power and flow rates than the mono-polar equivalent. Keywords: Alkaline; Electrolysis stack; Hydrogen; Renewable energy; Monopolar; Bipolar.
1. Introduction Today, the generation of hydrogen using electrolysis process has been identified as a very important environmentally friendly method. The renewable energy sources, particularly wind, hydro and solar, can be integrated to generate hydrogen from electrolysis process, by making the whole system as a zeroemission system. Thus, it is important to enhance the performance of the electrolysis cell in order to generate hydrogen through electrolysis process. The performance of electrolysis cell is directly proportional to the properties of MEA and the distance between electrodes. The ohmic loss of the alkaline electrolysis cell is mainly induced by the electrical resistance of the electrolytes, which can be reduced by shortening the distance between the anode and cathode. If the distance between the anode and cathode is too small, it may diminish the performance of
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the cell. Thus, there should be an optimum distance between the anode and cathode. Hence, the main aim of this research work is to investigate the optimum parameters to enhance the performance of electrolysis cells and fabricate the monopolar and bipolar configuration of alkaline electrolysis stacks to compare the performance through 3-D modelling and experimental fabrication. Moreover, the manufacturing cost of an alkaline electrolysis cell is comparatively lower than other existing electrolysis processes such as Proton Exchange Membrane (PEM) and Solid Oxide Electrolysis (SOE). The working principle of an alkaline electrolysis cell is shown in Figure 1.
Figure 1. Working principle of single cell alkaline electrolyser [1]. The anode and cathode electrochemical reactions of alkaline electrolyser can be illustrated as follows [2], Cathode:
2 H 2O 2e 2OH H 2
(1)
Anode:
1 2OH H 2O 2e O2 2
(2)
Total:
1 2 H 2O H 2 O2 H 2O 2
(3)
The generated hydrogen from the above-mentioned reaction does not require further refinement since it has a purity level in between 95.5% - 99.9998% [3]. Furthermore, the electrical conductivity value of the feeding water should be below 5µS/cm in order to make sure the purity level of feeding water [4]. Some research studies are presented in the literature on alkaline electrolysis process regarding design, modelling and different base materials that are used to enhance the performance of the cell. The different kinds of electrolysis processes are integrated with renewable energy sources, particularly wind, hydro and solar to observe the performance of the system [5 – 9]. Hydrogen has been used as long-term energy storage for an integrated stand-alone renewable energy system [10,11]. In Ref [12-14], the static and dynamic model of alkaline electrolysis stacks is explained. Moreover, an electrical equivalent model for a proton exchange membrane (PEM) electrolyser has been developed in Ref [15]. Prior to that, the performance of electrolysis process can be enhanced by introducing new materials for electrolysis cell [16, 17]. A porous polymeric diaphragm is deployed in Ref [18] to minimize ohmic resistance by increasing the ionic conductivity of the alkaline electrolysis cell. Hydrogen and oxygen production of the alkaline electrolysis process is decoupled using nickel hydroxide as a redox mediator, which overcomes the gas-mixing issue by enhancing the purity level of product gas [19]. The performance of alkaline water electrolysis is significantly affected by the distance between electrodes,
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electrolyte concentration and operating temperature [20]. In Ref [21], the ohmic resistance of alkaline electrolysis cell has been minimized through effective cell design. Furthermore, to enhance efficiency by reducing ohmic resistance, the zero-gap alkaline electrolysis cell design for renewable energy storage as hydrogen gas is introduced and discussed in Ref [22]. Comparatively, the manufacturing cost of alkaline electrolysis cell is much lower when compared with other existing electrolysis processes. The possible reduction of the hydrogen production cost can be obtained when operating alkaline electrolysers in a discontinuous way, in order to benefit from low electricity prices [23]. The performance of alkaline electrolyser can be studied through a programmable power supply under different operating conditions [24]. Reference [25] presented the implementation and control of electrolysers to achieve high penetrations of renewable power. In ref [26], it is found that the highest level of energetic reliability can be achieved with relatively low battery capacity and hydrogen storage. Besides that, in a multiple renewable energy hybrid micro-grid power system, the real power balance is controlled by using electrolysis system’s dynamic control method while enhancing the operational capability to handle frequency fluctuation [27]. Prior to that, the energy efficiency considerations on monopolar and bipolar fused salt electrolysis cells are discussed in Ref [28] and the author observed that the voltage and the current efficiency of both stacks showed approximately the same value and order for the case, calculated in this study. In this research, we describe the design considerations of a complete high-performance finite gap single cell alkaline electrolyser using 3D printed plastic housing since 3D printing technology is able to reduce the manufacturing cost of electrolysis cells by reducing costs for the post machining [29]. At the beginning of this study, two designs of electrolysis cells were fabricated, and the respective performances were examined. Moreover, the electrolysis power and efficiency of the cell were examined to select the best performing cell. Prior to that, the purity level and the flow rate of product gas were measured to compare the performance of fabricated cells. Thereafter, the performance of the selected cell was enhanced through several experiments by changing the properties of electrodes and varying the distance between electrodes to reduce the ohmic loss of the cell. At last the monopolar and bipolar configuration of alkaline electrolysis stack were fabricated by using the best performing single cell electrolyser to compare their performance. Throughout this research work, the Solidworks and MATLAB/SIMULINK were used as design software and modelling tool respectively. 2. Materials and Methods 2.1. Electrolysis Cell Materials In this research, 3D printed plastic housing is deployed to fabricate the electrolysis stack since the 3D printing technology is able to reduce the manufacturing cost of electrolysis cell by reducing costs for the post machining (additive manufacturing). The manufacturing process of plastic parts is accomplished by using an in-house 3D printer which uses a resin that is cured to form a dense plastic part. The nickel foam with middle overpotential is used as electrodes of the cell while using a 30 wt% aqueous solution of potassium hydroxide (KOH) as an electrolyte to enhance the ionic conductivity of the solution. A separating diaphragm keeps the product gasses apart to maintain efficiency and safety. Moreover, the diaphragm must be permeable to water molecules and hydroxide ions. In this research, two different polymeric membranes such as Nafion 115 (N115) and Nafion 117 (N117) are adopted as a separating diaphragm. Furthermore, a nylon separator with some filter papers is used as separating diaphragm in some experiments to avoid a short circuit effect inside the cell. The efficiency of electrolysis process is enhanced by adding an electrocatalyst into the electrodes or separating diaphragm of the cell. The electrocatalyst is developed inside the laboratory and the process is done by dissolving the same amount of “Nafion perfluorinated resin solution” and “Platinum on activated charcoal” inside Ethanol solution. Sealing gasket plays an important role in this design, particularly to minimize the leakage of gases and electrolyte to the outer atmosphere and to avoid the mixing of product gases near the catalyst area. The rubber gasket is also made in the laboratory by mixing the same amount (50%) of Ray Tech magic rubber solutions.
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2.2. Method In this research work, the main task can be classified into two groups as design and modelling of the cell, depending on the components and validation of the observed results as shown in Figure 2. SolidWorks software was used in the design part, to design base plates and the intermediate base plate. Thereafter, the lab-based 3D printer (Project type) was used to create a 3D print of designed cells. The printer is based on fused deposition modelling (FDM) 3D printing technology [30] and utilized a resin that is cured to form a dense plastic part. When designing the base plates, dimensions are selected in order to maintain proper spacing between the inside layers of alkaline electrolyser. Hence, to reduce the gas flow resistance inside the cell and to increase efficiency of the stack, the spacing between layers are quite important. At the beginning, two different single-cell alkaline elecrolysers are fabricated and the cell performance was observed individually to select the best cell design for the stack. Thereafter, several experiments were conducted to increase the performance of the selected cell. Thus, the best performing cell is selected for the final stack. Moreover, monopolar and bipolar configurations of electrolysis stack are developed to compare the performance of both configurations. During the fabrication process, nickel coated stainless steel is used as a current collector plate. A ray tech magic rubber solution was used to make the gasket in order to maximize the sealing and prevent the leakage of alkaline electrolysis stack,.
Figure 2. Classification of the design and model While fabricating the alkaline electrolysis cell, a sandwiching of all the inside parts occurs in between the base plates to implement the cell as shown in Figure 3.
Figure 3. Basic parts of alkaline electrolysis cell
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The mathematical model was developed in the MATLAB/SIMULINK environment to validate the observed results. At first, the best performing single cell electrolyser was selected by comparing observed results with a mathematical model. Thereafter, monopolar and bipolar configurations of electrolysis stack are developed by using selected single cell electrolyser and the observed results are compared with a developed mathematical model to examine the performance of both configurations. 3. Design and fabrication of single cell alkaline electrolyser In this research study, two electrolysis cells were designed, and their performance was examined to identify the most suitable design. Figure 4 (a) shows the 3D model of the first design of electrolysis cell and several experiments have been implemented by using the first design to examine the performance of the cell. While doing the experiments with the first design, it was observed that the product gasses contain some amount of water vapor, which was reduced the purity level of product gas. Thus, in order to enhance the purity level of product gas by eliminating this problem, the intermediate storage tank was designed on the intermediate base plate as shown in Figure 5 (b) (second design). That storage tank was able to condense water vapor while increasing the purity level of product gas and the performance of the cell as well. Hence the second design was selected as the best performing electrolysis cell and it was deployed to fabricate both monopolar and bipolar configurations of alkaline electrolysis stacks.
(a)
(b)
Figure 4. (a) 3D Model of the first electrolysis cell, and (b) 3D Model of the second electrolysis cell The dense plastic was used as a material for the base plate since it is able to reduce the cost for post machining. The metallic materials for the cathode can be divided into three classes as metals with high overpotentials (Cd, Ti, Hg, Pb, Zn, Sn, etc.), metals with middle overpotential (Fe, Co, Ni, Cu, Au, Ag, W etc.) and metals with low overpotentials (Pt, Pd etc.). According to the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of the electrolysis process, nickel and its alloy can be used as electrodes of the cell [31], [32]. Thereafter the electrocatalyst was applied to electrodes to enhance the efficiency of the electrolysis process by reducing the activation energy for the electrochemical reaction. Furthermore, the catalyst layer consists of a Nafion® perfluorinated solution, a 10% platinum on activated charcoal and methanol. The electrolyte is an aqueous solution which contained either potassium hydroxide (KOH) or sodium hydroxide (NaOH) with a typical concentration of 20 - 40 wt%. The separating diaphragm (separating papers and nylon separator) separates the anode and cathode from each other. The nylon mesh makes sure that there is no short circuit affects inside the cell. At last, to increase mechanical sealing inside the cell, the rubber was used as a gasket material.
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4. Design and fabrication of alkaline electrolysis stacks According to the calculation, the single cell electrolyser generates a low amount of hydrogen gas per minute. Hence, in order to produce a high amount of hydrogen gas, alkaline electrolysis stacks are designed in this research work. The alkaline electrolysis stacks can be categorized into two groups as monopolar and bipolar configurations. In monopolar configuration, each electrolysis cell is connected in parallel to form a large module of electrolysis stack and in a bipolar configuration, each electrolysis cell is connected in series to form a large module of electrolysis stack. The basic Solidworks part of monopolar and bipolar configurations are illustrated in Figure 5 and Figure 6 respectively.
Figure 5. Basic SolidWorks parts of the monopolar alkaline electrolysis stack
Figure 6. Basic SolidWorks parts of the bipolar alkaline electrolysis stack
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5. Modelling of electrolysis cell The mathematical model of alkaline electrolysis cell is developed in MATLAB/SIMULINK environment considering electrochemical equations. Figure 7 shows the V-I characteristic curve (Polarization Curve) of the typical alkaline electrolysis cell where the cell voltage is enhanced by some primary sources that caused to reduce the efficiency of the cell [33]. Hence, the cell voltage is a function of the overvoltages and reversible voltage as defined in (4). Vcell Vrev Vact Vohm Vcon
(4)
Where, Vcell = cell voltage, Vrev = reversible cell voltage, Vact = activation overvoltage, Vohm = ohmic losses and Vcon = concentration overvoltage.
Figure 7. V-I Characteristic curve of typical electrolysis cell [34] The Vrev can be calculated using the Gibbs equation as illustrated in (5) [35]. G zFVrev
(5) mol−1)
Where ∆G is Gibbs energy (237.2 kJ mol−1) and F is Faraday’s Constant (96485 C [35]. The analytical value of z equals to 2. Moreover, the Vact can be defined by following expression in (6). Vact
t 2 t3 t1 T T s log I 1 A
(6)
Where, s, t1, t2, t3 are the coefficients for overvoltage on electrodes, I is the current density, T is the temperature of the cell and A is the electrode area respectively [35], [36]. The electrical resistance of the electrolytes causes to generate the ohmic loss (Vohm) which mitigates the performance of the cell by a considerable amount. However, for the optimum anode and cathode distance, the electrolysis cell is able to reduce the ohmic loss by increasing cell performance. This overvoltage can be determined as in (7) [33].
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Vohm
r1 r2T I A
(7)
Where r1, r2 are ohmic resistance parameters [35], [36]. The Vcon is neglected in this modelling part since it has extremely low value compared to the Vact and Vohm. Hence, equation (4) can be rewritten as in (8). Vcell Vrev Vact Vohm
(8)
5.1. Simulation background Table 1 illustrates the relevant parameters including units which are used in mathematical model. Reference [35] and [36] are explained in the modelling part of the electrolysis cell and most of the data available in the table are taken from that research article. Table 1. Details of constants for mathematical modelling [35] Name of Constants
Symbols
Units
Value
Reversible Voltage Area of Electrode Faraday's Constant Number of Electrons The Coefficient for overvoltage on electrodes Coefficients for overvoltage on electrodes
𝑉𝑟𝑒𝑣 A F Z s 𝑡1 𝑡2 𝑡3
V 𝑐𝑚2 𝐶𝑚𝑜𝑙 ―1 V 𝐴 ―1𝑚2 𝐴 ―1𝑚2 0𝐶 𝐴 ―1𝑚2 0𝐶
1.229 0.25 96485 2 0.185 1.002 8.424 247.3
Parameters related to ohmic resistance of the electrolyte
𝑟1 𝑟2
𝛺𝑚2 𝛺𝑚2 0𝐶 ―1
8.05e-5 -2.5e--7
5.2. Mathematical model of the electrolysis cell A developed mathematical model in MATLAB/SIMULINK environment is illustrated in Figure 8. The operating temperature (T) and the area of the cell (A) are selected as inputs during this modelling study. The V-I characteristic and P-I characteristic of alkaline electrolyser obtained by mathematical model, are shown in Figure 9 and Figure 10 respectively. Furthermore, cell voltage and overvoltage characteristics are illustrated in Figure 11.
Figure 8. Mathematical model of the alkaline electrolysis cell
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Figure 9. V-I characteristic of the alkaline electrolysis cell
Figure 10. P-I characteristic of the alkaline electrolysis cell
Figure 11. Cell voltage and overvoltages of the alkaline electrolysis cell
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6. Experimental results and discussion In this research work, twelve different experiments have been conducted. At the very beginning, nine different experiments were done in order to enhance the performance of a single cell alkaline electrolyser by changing the properties of Membrane Electrode Assembly (MEA) and electrode distance of the cell. Thereafter, three experiments have been deployed to examine the performance of electrolysis stacks and the schematic view of the experimental setup for stack testing is illustrated in Figure 12. This paper is providing more detail and extended results of our work reported in Ref [37].
Figure 12. Schematic view of the experimental setup for stack testing 6.1. Experiment I: Commencement of the experiment The main objective of this experiment is to investigate working principle of the electrolysis process. For the experiment 30 wt% aqueous solutions of the KOH is deployed and, around 2.5 V electric current is supplied through the electrodes that are immersed in an electrolyte solution. The separating papers and nylon separator are placed between the anode and cathode to build up the membrane electrode assembly (MEA). Thereafter, all the parts of MEA are tied together to diminish the ohmic overvoltage and immersed inside the electrolysis solution to observe the electrochemical reactions. 6.2. Experiment II: Electrodes with electrocatalyst. In this experiment, an electrocatalyst is applied to electrodes in order to enhance the rate of electrochemical reaction which is taken place near the electrodes. A bench setup, which is illustrated in Figure 12, is used in this experiment as in Experiment I, and an electrocatalyst is coated to the electrodes of Experiment II. That is the only difference between Experiment I and Experiment II. The performance of Experiment I and Experiment II is compared with a mathematical model to investigate the performance of both experiments as shown in Figure 13. The results imply that the electrolysis cell with electrocatalyst exhibits better performance since the electrocatalyst is able to reduce the activation energy for the electrochemical reaction by increasing electrolysis cell efficiency.
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Figure 13. Performance comparison of Experiment I and II with the mathematical model. 6.3. Experiment III: Collecting the product gas using the first design. The product gas Collection is one of the important aspects of this research work. Thus, the MEA which is developed in Experiment II has been placed inside the cell frame to collect and measure the product gas flow rate. The electrolysis cell frame is designed using Solidworks as a design flat form. The flow rate of product gas was measured by using 100 ml measuring flask as illustrated in Figure 12. 6.4. Experiment IV: Enhancement of purity level of product gas through second design The purity level of product gas is a quite important parameter while investigating the performance of the cell. Thus, in this experiment, the purity level of product gas is enhanced by designing an intermediate storage tank on top of the intermediate base plate. The intermediate storage tank is able to condense water vapor before entering to the gas container by increasing the efficiency of the cell. Nevertheless, in Experiment III, the purity level of product gas is quite low, owing to the presence of water vapor. That problem can be mitigated by using the second design and the performance comparison of Experiment III and Experiment IV with the mathematical model is illustrated in Figure 14.
Figure 14. Performance comparison of Experiment III and IV with the mathematical model According to the performance comparison curve, Experiment IV exhibits comparatively a better performance owing to the small electrode distance that diminishes the ohmic loss of the cell by
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increasing efficiency. Moreover, the intermediate storage tank is able to enhance the performance of electrolysis cell. 6.5. Experiment V: Effect from the Thickness of Electrodes In this experiment, the thickness of the electrodes was enhanced by adding porous nickel plates to MEA. Same bench setup was deployed as in Experiment I, and the required data for efficiency and power calculations were obtained (Tables 2 and 3). The performance comparison was done through the V-I characteristic curve which was generated from a mathematical model. Table 2. Flow rate calculation data of Experiment V Produced Gases Cell Current (A) Voltage of Cell (V) Hydrogen (𝐻2(𝑔)) Oxygen (𝑂2(𝑔))
2.5 3.14 2.5 3.14
2.03 2.1 2.06 2.1
Time (s) 301 260 672 554
Table 3. Calculated parameters of Experiment V Flow rate calculation Power and efficiency calculation Variable Current of the cell (A) 𝑛𝐻2 (mol/min) 𝑛𝑜2 (mol/min)
Value
Variable
3.14 9.7632 x 10 ―4 4.8816 x 10
―4
Value
Voltage of cell (V) 𝑃𝑇𝑜𝑡𝑎𝑙 (W)
2.1 6.594
𝑅𝑂ℎ𝑚𝑖𝑐 (Ω)
0.207
𝑉𝐻2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛)
24.017
𝑃𝑂ℎ𝑚𝑖𝑐 (W)
2.0433
𝑉𝑂2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛)
12.008
𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (W)
4.5507
𝑉𝐻2(𝑔) (Exp)(𝑐𝑚 /𝑚𝑖𝑛)
23.077
𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (%)
69.01
𝑉𝑂2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
10.83
𝜂𝑒𝑛𝑒𝑟𝑔𝑦 (%)
70.5
3
6.6. Experiment VI: Effect of distance between electrodes The distance between electrodes of electrolysis cell is a critical fact affecting to the electrolysis performance, particularly power and efficiency. The performance of electrolysis cell could be increased by keeping an optimum distance between the anode and cathode while reducing the ohmic loss inside the cell. In order to investigate the effect of electrode distance for electrolysis process, three experiments were conducted while varying the distance between electrodes as 2.5mm, 1.9mm and 1.3 mm respectively. Figure 15 shows the bench setup of Experiment VI. The V-I characteristic curves of each experiment were drawn by considering experimental data and compared with mathematical model to examine the performance of the cell as shown in Figure 16. According to the observations of this experiment, a 1.3 mm thick electrode assembly exhibits better performance compared with other two experiments.
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Figure 15. Bench setup of Experiment VI
Figure 16. Comparison of the Experiments (Electrode gap 2.5mm, 1.9 mm, 1.3mm) with the mathematical model The results imply that the efficiency of electrolysis cell could be enhanced by decreasing electrode distance further since the electrode distance is directly proportional to the ohmic loss. Consequently, the ohmic loss can be diminished by reducing the electrode distance. Thus, the electrolysis cell with a 1.3mm thick electrode assembly has better performance compared to other experiments. 6.7. Experiment VII: Electrolysis cell with polymeric membrane separator In this experiment, Nafion 115 and Nafion 117 polymeric membranes were deployed as separating diaphragms respectively. According to the observed data, the Nafion 115 gave a good performance compared to the Nafion 117. Nevertheless, the performance of both polymeric membranes is much lower than when compared with other experiments. The performance comparison of both polymeric membranes with a mathematical model is shown in Figure 17.
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Figure 17. Performance comparison of both polymeric membranes with the mathematical model The performance comparison curve shows that the overvoltage losses of both polymeric membranes are extremely high compared with other experiments. The polymeric membranes are permeable to the water molecules and hydroxide ions. Nevertheless, the permeability resistance of membranes is higher than the electrode assembly of previous experiments. 6.8. Experiment VIII: Electrodes with porous stainless-steel plates To reduce the ohmic overvoltage, electrodes of the cell were replaced by porous stainless-steel plates and the relevant data was observed as in previous experiments by keeping the electrode gap as 1.3 mm. The observed data and calculated parameters are illustrated in Tables 4 and 5 respectively. Table 4. Flow rate calculation data of the Experiment VIII Produced Gases Cell Current (A) Voltage of Cell (V) Hydrogen (𝐻2(𝑔)) Oxygen (𝑂2(𝑔))
2.5 3.1 2.5 3.14
1.98 2.04 1.98 2.04
Time (s) 300 250 588 505
Table 5. Calculated parameters of the Experiment VIII Flow rate calculation Power and efficiency calculation Variable
Value
Current of the cell (A) 𝑛𝐻2 (mol/min)
9.7632 x 10
𝑛𝑜2 (mol/min)
Variable Voltage of cell (V) 𝑃𝑇𝑜𝑡𝑎𝑙 (W)
6.4056
4.8816 x 10 ―4
𝑅𝑂ℎ𝑚𝑖𝑐 (Ω)
0.194
23.017
𝑃𝑂ℎ𝑚𝑖𝑐 (W)
1.914
𝑉𝑂2(𝑔) (Theo)(𝑐𝑚 /𝑚𝑖𝑛)
12.01
𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (W)
4.4921
𝑉𝐻2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
24
𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (%)
70.13
𝑉𝑂2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
11.88
𝜂𝑒𝑛𝑒𝑟𝑔𝑦 (%)
72.5
𝑉𝐻2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛) 3
3.14
Value
―4
2.05
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6.9. Experiment IX: Electrolysis cell with fine nylon mesh In this experiment, one nylon mesh and two filter papers were adopted as separating diaphragms and, the same procedure was continued to observe the required data by keeping the gap between electrodes as 1.3 mm. Tables 6 and 7 shows the observed data and calculated parameters of Experiment IX. Table 6. Flow rate calculation data of the Experiment IX Produced Gases Cell Current (A) Voltage of Cell (V) Hydrogen (𝐻2(𝑔)) Oxygen (𝑂2(𝑔))
2.5 3.1 2.5 3.14
2 2.05 2 2.05
Time (s) 334 254 613 551
Table 7. Calculated parameters of the Experiment IX Flow rate calculation Power and efficiency calculation Variable
Value
Variable
Current of the cell (A) 𝑛𝐻2 (mol/min)
9.6388 x 10
𝑛𝑜2 (mol/min)
4.8194 x 10 ―4
𝑅𝑂ℎ𝑚𝑖𝑐 (Ω)
0.2
𝑉𝐻2(𝑔) (Theo)(𝑐𝑚 /𝑚𝑖𝑛)
23.71
𝑃𝑂ℎ𝑚𝑖𝑐 (W)
1.922
𝑉𝑂2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛)
11.86
𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (W)
4.433
𝑉𝐻2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
23.62
𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (%)
69.8
𝑉𝑂2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
10.89
𝜂𝑒𝑛𝑒𝑟𝑔𝑦 (%)
72.2
3
3.1 ―4
Voltage of cell (V) 𝑃𝑇𝑜𝑡𝑎𝑙 (W)
Value 2.05 6.355
Figure 18 is illustrated combinations of V-I characteristic curves of the Experiments V, VIII, and IX with the mathematical model as a comparison of particular experiments.
Figure 18. Comparison of the Experiments V, VIII, and IX with the mathematical model The performance comparison curve implies that the electrolysis cell in Experiment VIII has extremely better performance when compared with other experiments. In that experiment, we used
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porous electrodes by keeping electrode distance as 1.3 mm as in Experiment VI. The porous electrodes are able to reduce the internal losses of the electrolysis cell by increasing efficiency. 6.10. Experiment X: Implementation of monopolar electrolysis stack. The single cell alkaline electrolyser in the Experiment VIII is deployed while fabricating the monopolar electrolysis stack owing to the great performance compared with the other experiments. the stack consists of three single cells as illustrated in Figure 19.
Figure 19. Bench setup of Experiment X 6.11. Experiment XI: Implementation of bipolar electrolysis stack. Same as in Experiment X, the single cell alkaline electrolyser in the Experiment VIII is adopted while fabricating the bipolar electrolysis stack as well, owing to the great performance compared with the other experiments. the stack consists of three single cells as illustrated in Figure 20.
Figure 20. Bench setup of Experiment XI
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The performance comparison of monopolar and bipolar configuration of alkaline electrolysis stacks is one of the major objectives of this study. Hence, the V-I characteristic curve of both electrolysis stacks and mathematical model are drawn in the same axis to have a better idea about performance compression as shown in Figure 21.
Figure 21. Performance comparison of monopolar and bipolar configuration with the mathematical model According to the plot of performance comparison between monopolar and bipolar configuration of electrolysis stack, bipolar stack exhibits better performance than monopolar stack since the bipolar configuration is more compact and has a small ohmic loss compared to the monopolar equivalent. 6.12. Experiment XII: AC Impedance Test In this study, it is extremely important to obtain the impedance and the ohmic resistance of the cell. There are several methods available to analyze the AC impedance of the electrolysis cell such as the electrochemical impedance spectroscopy (EIS), the current sweep and the current interruption. Normally, the current sweep and the current interruption method cannot be used to determine the impedance of the cell during operation [38]. Consequently, the EIS is used to measure the ohmic resistance of the cell during the operation in this research study. The bench set up of Experiment XII is shown in Figure 22.
Figure 22. Bench setup for AC - impedance test
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The impedance is a complex quantity with a real and imaginary part. The real part of the impedance is directly proportional to the ohmic resistance of the electrolysis cell. The Nyquist plot is commonly used to display the impedance of the system. The AC impedance plots of monopolar and bipolar configuration of the electrolysis stack are illustrated in Figure 23 and Figure 24respectively.
Figure 23. AC - impedance plot of the monopolar electrolysis stack
Figure 24. AC - impedance plot of the bipolar electrolysis stack According to the impedance plot of the monopolar and bipolar electrolysis stack, the ohmic resistance of the stack is observed as 0.714 Ω and 0.621 Ω respectively. The bench set up of Experiment XII is shown in Figure 24. In this research study, a comparison of bipolar and monopolar electrolysis stacks was presented in detail. First, two different single cell electrolysers were developed and its performance was examined to select the best performing cell for the stack design. According to the observations, the second design of single cell electrolyser was selected for the stack design because of its performance. Consequently, electrolysis efficiencies of the first and the second designs were 67.9% and 69.5% respectively. Prior to that, the purity level of product gas was also considered while selecting the best performing cell. Especially, in the second design, the purity level of product gas was enhanced by an intermediate storage tank that was able to condense water vapor before entering to the gas container. The gas mixing issue of this research work was eliminated by introducing polymeric membrane and filter papers for MEA which enhance the purity level of product gas by a considerable amount. The purity level of product gas was enhanced in Ref [19], by decoupling anode and cathode through nickel hydroxide as a redox mediator to overcomes the gas-mixing issue. Thereafter, the electrolysis efficiency of the second design was enhanced further from 69.5% to 72.5% (Experiment VIII) through several experiments by changing materials and electrode distance, before using it for stack design. Thus, the bipolar and monopolar configuration of alkaline electrolysis stack (3-cell stack) was developed by using the electrolysis cell which was developed under Experiment VIII. Consequently, the electrolysis efficiency of the bipolar
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and monopolar stack was calculated as 67.9% and 69.5% respectively. The results imply that the designed bipolar stack has a better performance in terms of efficiency, power and flow rates than the monopolar equivalent. The same procedure was adopted in Ref [39], for developing and testing of a highly efficient proton exchange membrane (PEM) electrolyzer stack. In that research study, the author emphasized that future studies need to be done to reduce manufacturing cost by employing cheaper materials or coating. In this research work, as a cell material, we employed high quality, durable dense plastic and 3D printing technology to reduce the cost associated with post machining process. The comparable bipolar and monopolar connection of capacitive deionization stack in NaCl treatment was discussed in previous studies [40]. In that study, both bipolar and monopolar configuration of electrolysis stack was designed, and they were compared for the influence of the voltage distribution depending on the cell structure. The bipolar circuit structure was more favorable to adsorb the ions electrostatically to the electrode surface although there are voltage balancing problems between each cell of the stack. The bipolar and unipolar configuration of alkaline electrolysis stack, solid polymer electrolyte, and vapor-phase technologies are compared in Ref [41]. It is concluded that alkaline electrolysis at temperatures below 150°C will be economically preferred over the next decade, while vapor-phase electrolysis is a development of significant but longer-term interest. Thus, the manufacturing cost of the alkaline electrolysis stack was comparatively lower than other existing technology and the bipolar stack exhibit better performance when compared with the monopolar stack. Nevertheless, optimization of the stack performance by using different materials for electrodes requires further research. 7. Conclusion In this research, we have enhanced the performance of the electrolysis process by decreasing the distance between the anode and cathode, while changing the properties of the Membrane Electrode Assembly (MEA). The main aim of this research was to design and fabricating of mono-polar and bi-polar configuration of alkaline electrolysis stacks to compare their performance through 3-D modelling and experimental fabrication. The first four experiments have been used as benchmark tests to determine the best single cell configuration which was able to provide adequate product gas flow rate with high purity level. Thus, two designs of single cell alkaline electrolysis cells have been deployed, and the second design has been selected as the best performing cell after considering their performance separately. The different experiments were carried out to improve the performance of the selected cell and Experiment VIII has been selected as the best performing cell after considering the performance comparison graph with the mathematical model. During that experiment, the distance between porous electrodes was set to 1.3 mm to reduce the ohmic overvoltage inside the cell, by increasing the efficiency and power. According to the observations of that experiment, the practical values of hydrogen gas flow rate, oxygen gas flow rate and electrolysis efficiency have been calculated as 24 cm3/min, 11.88 cm3/min and 70.13% respectively. The bi-polar and mono-polar configurations of electrolysis stack have been fabricated by using the electrolysis cell, which has been developed in Experiment VIII, and the rest of the experiments have been implemented to investigate the performance of fabricated electrolysis stacks. The observation implies that the bi-polar electrolysis stack exhibited better performance than the mono-polar electrolysis stack. Furthermore, the hydrogen gas flow rate, the oxygen gas flow rate and the electrolysis efficiency have been calculated for bi-polar configuration, as 71.43 cm3/min, 35.96 cm3/min and 69.88 % respectively. Moreover, the bi-polar stack is more compact than the mono-polar stack and the AC impedance tests have been implemented to obtain the exact values of ohmic loss inside both stacks. Nafion has been deployed as a catalyst layer to enhance the rate of HER and OER. Particularly, the Nafion is an acid and reacts with the alkaline solution by reducing the lifetime of the cell. Nevertheless, the life expectancy of the cell was more than adequate for experimental purposes. In the future, we intend to develop a zero-gap electrolysis cell with different catalyst layers to reduce the ohmic loss further and, an optical sensing technology to be integrated for observing the optimum distance between the electrodes quite accurately. It is obvious that all the observations of this research study indicate that the designed bi-polar stack has a better performance in terms of efficiency, power and flow rates than the mono-polar equivalent.
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References [1] A. Ursua, L. M. Gandia, and P. Sanchis, “Hydrogen production from water electrolysis: Current status and future trends,” Proceedings of the IEEE, vol. 100, no. 2, pp. 410-426, Feb. 2012. [2] K. Zeng and D. Zhang, “Recent progress in alkaline water electrolysis for hydrogen production and applications,” Progress in Energy and Combustion Science, vol. 36, no. 3, pp. 307-326, Jun. 2010. [3] Johanna Ivy, “Summary of electrolytic hydrogen production,” Milestone Completion Report, 2004. [4] L. Bertuccioli, A. Chan, D. Hart, F. Lehner, B. Madden, and E. Standen, “Study on development of water electrolysis in the EU,” Fuel Cells and Hydrogen Joint Undertaking, p. Final Report, Feb. 2014. [5] A. Ursa, I. San Martn, E. L. Barrios, and P. Sanchis, “Stand-alone operation of an alkaline water electrolyser fed by wind and photovoltaic systems,” International Journal of Hydrogen Energy, vol. 38, no. 35, pp. 14952-14967, Nov. 2013. [6] M. Kiaee, P. Chladek, D. Infield, and A. Cruden, “Utilisation of alkaline electrolysers to improve power system frequency stability with a high penetration of wind power,” IET Renewable Power Generation, vol. 8, no. 5, pp. 529-536, Jul. 2014. [7] W. Jiang, et al, “Identification and Power Electronic Module Design of a Solar Powered Hydrogen Electrolyser,” IEEE Asia-Pacific Power and Energy Engineering Conference, pp. 1-4. 2012. [8] M. Kiaee, A. Cruden, D. Infield, Y. Ma, T. G.Douglas, “The impact on the electrical grid of hydrogen production from alkaline electrolysers,” IEEE Universities Power Engineering Conference (UPEC), 45th International, pp. 1-6. 2010. [9] N. Gyawali and Y. Ohsawa, “Integrating fuel cell/Electrolyzer/Ultracapacitor system into a standalone Microhydro plant,” IEEE Transactions on Energy Conversion, vol. 25, no. 4, pp. 10921101, Dec. 2010. [10] M Kolhe, K. Agbossou, J. Hamelin, and T.K. Bose, “Analytical model for predicting the performance of photovoltaic array coupled with wind energy in a stand-alone renewable energy system based on hydrogen”, Renewable Energy (Elsevier), Vol.28, No.5, pp. 727-742, 2003. [11] K. Agbossou, M. Kolhe, J. Hamelin, and T.K. Bose, “Performance of stand-alone renewable energy system based on energy storage as hydrogen”, IEEE Trans on Energy Conversion, Vol. 19, Issue 3, pp. 633- 640, Sept. 2004. DOI: 10.1109/TEC.2004.827719. [12] M. ul-Karim1, M. T. Iqbal, “Dynamic modeling and simulation of alkaline type electrolyzers,” Electrical and Computer Engineering, CCECE’09. IEEE Canadian Conference, pp. 711-715. 2009. [13] K. Belmokhtar, M. L. Doumbia, K. Agbossou, “Dynamic model of an alkaline electrolyzer based an artificial neural network,” Ecological Vehicles and Renewable Energies (EVER), IEEE 8th International Conference and Exhibition, pp. 1-4. 2013. [14] I. D. de Cerio Mendaza and B. Bak-Jensen, “Alkaline electrolyzer and V2G system DIgSILENT models for demand response analysis in future distribution networks,” PowerTech (POWERTECH), IEEE Grenoble, pp. 1-8. 2013. [15] O. Atlam, M. Kolhe, “Equivalent electrical model for proton exchange membrane electrolyser”, Energy Conversion and Management (Elsevier), vol. 52, issues 8-9, pp. 2952-2957, 2011. DOI 10.1016/j.enconman.2011.04.007. [16] A. Cruden, D. Infield, M. Kiaee, T. G. Douglas, and A. Roy, “Development of new materials for alkaline electrolysers and investigation of the potential electrolysis impact on the electrical grid,” Renewable Energy, vol. 49, pp. 53-57, Jan. 2013. [17] Ph. Vermeiren, W. Adriansens, J. P. Moreels, R. Leysen, “Evaluation of the ZirfonR separator for use in alkaline water electrolysis and Ni-H 2 batteries,” International Journal of Hydrogen Energy, 23.5: 321-324.1998. [18] J. Kerres, G. Eigenberger, S. Reichle, V. Schramm, K. Hetzel, W. Schnurnberger, and I. Seybold, “Advanced alkaline electrolysis with porous polymeric diaphragms.” Desalination, 104(1-2), pp.47-57, 1996.
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[19] L. Chen, X. Dong, Y. Wang, and Y. Xia, “Separating hydrogen and oxygen evolution in alkaline water electrolysis using nickel hydroxide” Nature Communications, 7(1), 2016. [20] Md S. Opu, "Effect of operating parameters on performance of alkaline water electrolysis," Int. J. of Thermal & Environmental Engineering, vol. Volume 9, no. No. 2, pp. 53-60, 2015. [21] R. Phillips, A. Edwards, B. Rome, D. Jones, and C. Dunnill, “Minimising the ohmic resistance of an alkaline electrolysis cell through effective cell design,” International Journal of Hydrogen Energy, 42(38), pp.23986-23994, 2017. [22] R. Phillips and C. Dunnill, “Zero gap alkaline electrolysis cell design for renewable energy storage as hydrogen gas,” RSC Advances, 6(102), pp.100643-100651, 2016. [23] C. Mansilla, S. Dautremont, B. Shoai Tehrani, G. Cotin, S. Avril, and E. Burkhalter, “Reducing the hydrogen production cost by operating alkaline electrolysis as a discontinuous process in the French market context,” International Journal of Hydrogen Energy, 36(11), pp.6407-6413, 2011. [24] A. Ursa, I. S. Martn, and P. Sanchis, "Design of a programmable power supply to study the performance of an alkaline electrolyser under different operating conditions," 2nd IEEE Energy Conference & Exhibition, 2012. [25] E. Troncoso and M. Newborough, "Implementation and control of electrolysers to achieve high penetrations of renewable power," International Journal of Hydrogen Energy, vol. 32, no. 13, pp. 2253-2268, Sep. 2007. [26] P. C. Ghosh, B. Emonts, H. Janben, J. Mergel, and D. Stolten, "Ten years of operational experience with a hydrogen-based renewable energy supply system," Solar Energy, vol. 75, no. 6, pp. 469-478, Dec. 2003. [27] X. Li, Y.J. Song, and S.B. Han, "Frequency control in micro-grid power system combined with electrolyser system and fuzzy PI controller," Journal of Power Sources, vol. 180, no. 1, pp. 468-475, May 2008. [28] T. Beck, I. Rousar, and J. Thonstad, “Energy efficiency considerations on monopolar vs bipolar fused salt electrolysis cells,” Metallurgical and Materials Transactions B, 25(5), pp.661-668, 1994. [29] Y. Sanath K. De Silva, M.J.M.A. Rasul, P. Middleton and M. Kolhe, “PEM FC Single Cell Based on a 3-D Printed Plastic Housing and Experimental Validation with the Mathematical Model”, Energy Procedia, 144, pp.63-74, 2018. [30] Usglobalimages.stratasys.com. (2018). [online] Available at: http://usglobalimages.stratasys.com/Main/Files/Machine_Spec_Sheets/PSS_ FDM_Dim1200es.pdf [Accessed 15 May 2018]. [31] Md Mamoon Rashid, Mohammed K. Al Mesfer, Hamid Naseem, Mohd Danish, “Hydrogen production by water electrolysis: a review of alkaline water electrolysis, pem water electrolysis and high temperature water electrolysis,” International Journal of Engineering and Advanced Technology (IJEAT), vol. Volume-4, no. Issue-3, pp. 2249-8958, Feb. 2015. [32] M. Wang, Z. Wang, X. Gong, and Z. Guo, “The intensification technologies to water electrolysis for hydrogen production - A review,” Renewable and Sustainable Energy Reviews, vol. 29, pp. 573-588, Jan. 2014. [33] A. Ursa and P. Sanchis, “Static-dynamic modelling of the electrical behavior of a commercial advanced alkaline water electrolyser,” International Journal of Hydrogen Energy, vol. 37, no. 24, pp. 18598-18614, Dec. 2012 [34] Isao. Abe, “Alkaline Water Electrolysis”. Energy Carriers and Conversation Systems - Vol. 1. Chiba, Japan: Encyclopedia of Life Support System (EOLSS) [35] A. S. Tijani, N. A. B. Yusup, and A. H. A. Rahim, “Mathematical Modelling and simulation analysis of advanced alkaline Electrolyzer system for hydrogen production,” Procedia Technology, vol. 15, pp. 798-806, 2014. [36] O. Ulleberg, “Modeling of advanced alkaline electrolyzers: A system simulation approach,” International Journal of Hydrogen Energy, vol. 28, no. 1, pp. 21-33, Jan. 2003. [37] Y. S. K. De Silva, P. H. Middleton, M. Kolhe, “Performance Analysis of Single Cell Alkaline Electrolyser Using Mathematical Model”, The 5th International Symposium on Hydrogen
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Journal Pre-proof Author Contribution Statements I, Y. Sanath K. De Silva attached to the Faculty of Engineering and Science, University of Agder, Norway, as an MSc student. Presented paper is a part of my master study. Prof. Peter Hugh Middleton and Prof. Mohan Lal Kolhe were my supervisors. The presented idea developed by Prof. Peter Hugh Middleton and Prof. Mohan Lal Kolhe developed the theory and performed the computations. All authors discussed the results and contributed to the final manuscript. Y. Sanath K. De Silva carried out the experiment and wrote the manuscript with support from Prof. Peter Hugh Middleton and Prof. Mohan Lal Kolhe. All authors contributed to the final version of the manuscript. Prof. Peter Hugh Middleton supervised the project. All authors provided critical feedback and helped shape the research, analysis and manuscript and discussed the results and commented on the manuscript.
Journal Pre-proof Declaration of Interest Statements
The University of Agder, Norway has equity ownership in the paper of “Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication”. The University of Agder, at Norway may benefit from this interest, if the research article is successfully published in this journal. The terms of this arrangement have been reviewed and approved by the University of Agder, Norway in accordance with its policy on objectivity in research.
Journal Pre-proof Highlights
Alkaline electrolysis configuration (i.e. mono-polar and bi-polar) modelling for performance analysis.
Membrane electrode assembly (MEA) configuration effect on electrolysis performance.
Improvement of electrolysis performance with bi-polar stack compared to mono-polar equivalent.
Y. Sanath K. De Silva, Peter Hugh Middleton, Mohan Kolhe PII:
S0960-1481(19)31959-7
DOI:
https://doi.org/10.1016/j.renene.2019.12.087
Reference:
RENE 12796
To appear in:
Renewable Energy
Received Date:
15 June 2019
Accepted Date:
19 December 2019
Please cite this article as: Y. Sanath K. De Silva, Peter Hugh Middleton, Mohan Kolhe, Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication., Renewable Energy (2019), https://doi.org/10. 1016/j.renene.2019.12.087
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof Title: Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication.
Author names and affiliations: Y. Sanath K. De Silvaa, Peter Hugh Middletonb and Mohan Kolheb a,bFaculty
of Engineering and Science, University of Agder (UiA), PO Box 422, NO 4604, Kristiansand, Norway. [email protected],[email protected]
and [email protected]
Corresponding author: Y. Sanath K. De Silva, Email: [email protected] Present Address: Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, University of Ruhuna, Hapugala, Galle.
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Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication. Y. Sanath K. De Silva, Peter Hugh Middleton, Mohan Lal Kolhe. Faculty of Engineering and Science, University of Agder, PO Box 422, NO 4604, Kristiansand, Norway. E-mails: [email protected], [email protected], [email protected] Abstract. Generation of hydrogen using electrolysis process with integrated renewable energy sources is highly important especially in environmental aspects. In this paper, we demonstrate that the enhancement of electrolysis performance of alkaline electrolysis stacks by diminishing the distance between electrodes, while changing the properties of the Membrane Electrode Assembly (MEA). Prior to that, the performances of mono-polar and bi-polar configurations of alkaline electrolysis stack are compared through 3-D modelling and experimental fabrication. At first, two different single cell alkaline electrolysers are designed using SolidWorks as a design software and the designed cell has been fabricated using an in-house 3D printer, to avoid post machining processes. Thereafter, the best performing cell is selected by considering the performance of both designs through different experiments. Finally, the performance of the selected cell is enhanced by changing the distance between electrodes and properties of MEA. Thus, the best performing cell has been selected for the fabrication process of mono-polar and bi-polar electrolysis stack. At last, both mono-polar and bi-polar configurations of alkaline electrolysis stacks are designed and implemented to compare the electrolysis performance of both configurations by maintaining minimum electrode distance. The results imply that the electrolysis performance of the cell can be enhanced by reducing the distance between electrodes, and the designed bi-polar stack has a better performance in terms of the efficiency, power and flow rates than the mono-polar equivalent. Keywords: Alkaline; Electrolysis stack; Hydrogen; Renewable energy; Monopolar; Bipolar.
1. Introduction Today, the generation of hydrogen using electrolysis process has been identified as a very important environmentally friendly method. The renewable energy sources, particularly wind, hydro and solar, can be integrated to generate hydrogen from electrolysis process, by making the whole system as a zeroemission system. Thus, it is important to enhance the performance of the electrolysis cell in order to generate hydrogen through electrolysis process. The performance of electrolysis cell is directly proportional to the properties of MEA and the distance between electrodes. The ohmic loss of the alkaline electrolysis cell is mainly induced by the electrical resistance of the electrolytes, which can be reduced by shortening the distance between the anode and cathode. If the distance between the anode and cathode is too small, it may diminish the performance of
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the cell. Thus, there should be an optimum distance between the anode and cathode. Hence, the main aim of this research work is to investigate the optimum parameters to enhance the performance of electrolysis cells and fabricate the monopolar and bipolar configuration of alkaline electrolysis stacks to compare the performance through 3-D modelling and experimental fabrication. Moreover, the manufacturing cost of an alkaline electrolysis cell is comparatively lower than other existing electrolysis processes such as Proton Exchange Membrane (PEM) and Solid Oxide Electrolysis (SOE). The working principle of an alkaline electrolysis cell is shown in Figure 1.
Figure 1. Working principle of single cell alkaline electrolyser [1]. The anode and cathode electrochemical reactions of alkaline electrolyser can be illustrated as follows [2], Cathode:
2 H 2O 2e 2OH H 2
(1)
Anode:
1 2OH H 2O 2e O2 2
(2)
Total:
1 2 H 2O H 2 O2 H 2O 2
(3)
The generated hydrogen from the above-mentioned reaction does not require further refinement since it has a purity level in between 95.5% - 99.9998% [3]. Furthermore, the electrical conductivity value of the feeding water should be below 5µS/cm in order to make sure the purity level of feeding water [4]. Some research studies are presented in the literature on alkaline electrolysis process regarding design, modelling and different base materials that are used to enhance the performance of the cell. The different kinds of electrolysis processes are integrated with renewable energy sources, particularly wind, hydro and solar to observe the performance of the system [5 – 9]. Hydrogen has been used as long-term energy storage for an integrated stand-alone renewable energy system [10,11]. In Ref [12-14], the static and dynamic model of alkaline electrolysis stacks is explained. Moreover, an electrical equivalent model for a proton exchange membrane (PEM) electrolyser has been developed in Ref [15]. Prior to that, the performance of electrolysis process can be enhanced by introducing new materials for electrolysis cell [16, 17]. A porous polymeric diaphragm is deployed in Ref [18] to minimize ohmic resistance by increasing the ionic conductivity of the alkaline electrolysis cell. Hydrogen and oxygen production of the alkaline electrolysis process is decoupled using nickel hydroxide as a redox mediator, which overcomes the gas-mixing issue by enhancing the purity level of product gas [19]. The performance of alkaline water electrolysis is significantly affected by the distance between electrodes,
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electrolyte concentration and operating temperature [20]. In Ref [21], the ohmic resistance of alkaline electrolysis cell has been minimized through effective cell design. Furthermore, to enhance efficiency by reducing ohmic resistance, the zero-gap alkaline electrolysis cell design for renewable energy storage as hydrogen gas is introduced and discussed in Ref [22]. Comparatively, the manufacturing cost of alkaline electrolysis cell is much lower when compared with other existing electrolysis processes. The possible reduction of the hydrogen production cost can be obtained when operating alkaline electrolysers in a discontinuous way, in order to benefit from low electricity prices [23]. The performance of alkaline electrolyser can be studied through a programmable power supply under different operating conditions [24]. Reference [25] presented the implementation and control of electrolysers to achieve high penetrations of renewable power. In ref [26], it is found that the highest level of energetic reliability can be achieved with relatively low battery capacity and hydrogen storage. Besides that, in a multiple renewable energy hybrid micro-grid power system, the real power balance is controlled by using electrolysis system’s dynamic control method while enhancing the operational capability to handle frequency fluctuation [27]. Prior to that, the energy efficiency considerations on monopolar and bipolar fused salt electrolysis cells are discussed in Ref [28] and the author observed that the voltage and the current efficiency of both stacks showed approximately the same value and order for the case, calculated in this study. In this research, we describe the design considerations of a complete high-performance finite gap single cell alkaline electrolyser using 3D printed plastic housing since 3D printing technology is able to reduce the manufacturing cost of electrolysis cells by reducing costs for the post machining [29]. At the beginning of this study, two designs of electrolysis cells were fabricated, and the respective performances were examined. Moreover, the electrolysis power and efficiency of the cell were examined to select the best performing cell. Prior to that, the purity level and the flow rate of product gas were measured to compare the performance of fabricated cells. Thereafter, the performance of the selected cell was enhanced through several experiments by changing the properties of electrodes and varying the distance between electrodes to reduce the ohmic loss of the cell. At last the monopolar and bipolar configuration of alkaline electrolysis stack were fabricated by using the best performing single cell electrolyser to compare their performance. Throughout this research work, the Solidworks and MATLAB/SIMULINK were used as design software and modelling tool respectively. 2. Materials and Methods 2.1. Electrolysis Cell Materials In this research, 3D printed plastic housing is deployed to fabricate the electrolysis stack since the 3D printing technology is able to reduce the manufacturing cost of electrolysis cell by reducing costs for the post machining (additive manufacturing). The manufacturing process of plastic parts is accomplished by using an in-house 3D printer which uses a resin that is cured to form a dense plastic part. The nickel foam with middle overpotential is used as electrodes of the cell while using a 30 wt% aqueous solution of potassium hydroxide (KOH) as an electrolyte to enhance the ionic conductivity of the solution. A separating diaphragm keeps the product gasses apart to maintain efficiency and safety. Moreover, the diaphragm must be permeable to water molecules and hydroxide ions. In this research, two different polymeric membranes such as Nafion 115 (N115) and Nafion 117 (N117) are adopted as a separating diaphragm. Furthermore, a nylon separator with some filter papers is used as separating diaphragm in some experiments to avoid a short circuit effect inside the cell. The efficiency of electrolysis process is enhanced by adding an electrocatalyst into the electrodes or separating diaphragm of the cell. The electrocatalyst is developed inside the laboratory and the process is done by dissolving the same amount of “Nafion perfluorinated resin solution” and “Platinum on activated charcoal” inside Ethanol solution. Sealing gasket plays an important role in this design, particularly to minimize the leakage of gases and electrolyte to the outer atmosphere and to avoid the mixing of product gases near the catalyst area. The rubber gasket is also made in the laboratory by mixing the same amount (50%) of Ray Tech magic rubber solutions.
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2.2. Method In this research work, the main task can be classified into two groups as design and modelling of the cell, depending on the components and validation of the observed results as shown in Figure 2. SolidWorks software was used in the design part, to design base plates and the intermediate base plate. Thereafter, the lab-based 3D printer (Project type) was used to create a 3D print of designed cells. The printer is based on fused deposition modelling (FDM) 3D printing technology [30] and utilized a resin that is cured to form a dense plastic part. When designing the base plates, dimensions are selected in order to maintain proper spacing between the inside layers of alkaline electrolyser. Hence, to reduce the gas flow resistance inside the cell and to increase efficiency of the stack, the spacing between layers are quite important. At the beginning, two different single-cell alkaline elecrolysers are fabricated and the cell performance was observed individually to select the best cell design for the stack. Thereafter, several experiments were conducted to increase the performance of the selected cell. Thus, the best performing cell is selected for the final stack. Moreover, monopolar and bipolar configurations of electrolysis stack are developed to compare the performance of both configurations. During the fabrication process, nickel coated stainless steel is used as a current collector plate. A ray tech magic rubber solution was used to make the gasket in order to maximize the sealing and prevent the leakage of alkaline electrolysis stack,.
Figure 2. Classification of the design and model While fabricating the alkaline electrolysis cell, a sandwiching of all the inside parts occurs in between the base plates to implement the cell as shown in Figure 3.
Figure 3. Basic parts of alkaline electrolysis cell
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The mathematical model was developed in the MATLAB/SIMULINK environment to validate the observed results. At first, the best performing single cell electrolyser was selected by comparing observed results with a mathematical model. Thereafter, monopolar and bipolar configurations of electrolysis stack are developed by using selected single cell electrolyser and the observed results are compared with a developed mathematical model to examine the performance of both configurations. 3. Design and fabrication of single cell alkaline electrolyser In this research study, two electrolysis cells were designed, and their performance was examined to identify the most suitable design. Figure 4 (a) shows the 3D model of the first design of electrolysis cell and several experiments have been implemented by using the first design to examine the performance of the cell. While doing the experiments with the first design, it was observed that the product gasses contain some amount of water vapor, which was reduced the purity level of product gas. Thus, in order to enhance the purity level of product gas by eliminating this problem, the intermediate storage tank was designed on the intermediate base plate as shown in Figure 5 (b) (second design). That storage tank was able to condense water vapor while increasing the purity level of product gas and the performance of the cell as well. Hence the second design was selected as the best performing electrolysis cell and it was deployed to fabricate both monopolar and bipolar configurations of alkaline electrolysis stacks.
(a)
(b)
Figure 4. (a) 3D Model of the first electrolysis cell, and (b) 3D Model of the second electrolysis cell The dense plastic was used as a material for the base plate since it is able to reduce the cost for post machining. The metallic materials for the cathode can be divided into three classes as metals with high overpotentials (Cd, Ti, Hg, Pb, Zn, Sn, etc.), metals with middle overpotential (Fe, Co, Ni, Cu, Au, Ag, W etc.) and metals with low overpotentials (Pt, Pd etc.). According to the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of the electrolysis process, nickel and its alloy can be used as electrodes of the cell [31], [32]. Thereafter the electrocatalyst was applied to electrodes to enhance the efficiency of the electrolysis process by reducing the activation energy for the electrochemical reaction. Furthermore, the catalyst layer consists of a Nafion® perfluorinated solution, a 10% platinum on activated charcoal and methanol. The electrolyte is an aqueous solution which contained either potassium hydroxide (KOH) or sodium hydroxide (NaOH) with a typical concentration of 20 - 40 wt%. The separating diaphragm (separating papers and nylon separator) separates the anode and cathode from each other. The nylon mesh makes sure that there is no short circuit affects inside the cell. At last, to increase mechanical sealing inside the cell, the rubber was used as a gasket material.
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4. Design and fabrication of alkaline electrolysis stacks According to the calculation, the single cell electrolyser generates a low amount of hydrogen gas per minute. Hence, in order to produce a high amount of hydrogen gas, alkaline electrolysis stacks are designed in this research work. The alkaline electrolysis stacks can be categorized into two groups as monopolar and bipolar configurations. In monopolar configuration, each electrolysis cell is connected in parallel to form a large module of electrolysis stack and in a bipolar configuration, each electrolysis cell is connected in series to form a large module of electrolysis stack. The basic Solidworks part of monopolar and bipolar configurations are illustrated in Figure 5 and Figure 6 respectively.
Figure 5. Basic SolidWorks parts of the monopolar alkaline electrolysis stack
Figure 6. Basic SolidWorks parts of the bipolar alkaline electrolysis stack
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5. Modelling of electrolysis cell The mathematical model of alkaline electrolysis cell is developed in MATLAB/SIMULINK environment considering electrochemical equations. Figure 7 shows the V-I characteristic curve (Polarization Curve) of the typical alkaline electrolysis cell where the cell voltage is enhanced by some primary sources that caused to reduce the efficiency of the cell [33]. Hence, the cell voltage is a function of the overvoltages and reversible voltage as defined in (4). Vcell Vrev Vact Vohm Vcon
(4)
Where, Vcell = cell voltage, Vrev = reversible cell voltage, Vact = activation overvoltage, Vohm = ohmic losses and Vcon = concentration overvoltage.
Figure 7. V-I Characteristic curve of typical electrolysis cell [34] The Vrev can be calculated using the Gibbs equation as illustrated in (5) [35]. G zFVrev
(5) mol−1)
Where ∆G is Gibbs energy (237.2 kJ mol−1) and F is Faraday’s Constant (96485 C [35]. The analytical value of z equals to 2. Moreover, the Vact can be defined by following expression in (6). Vact
t 2 t3 t1 T T s log I 1 A
(6)
Where, s, t1, t2, t3 are the coefficients for overvoltage on electrodes, I is the current density, T is the temperature of the cell and A is the electrode area respectively [35], [36]. The electrical resistance of the electrolytes causes to generate the ohmic loss (Vohm) which mitigates the performance of the cell by a considerable amount. However, for the optimum anode and cathode distance, the electrolysis cell is able to reduce the ohmic loss by increasing cell performance. This overvoltage can be determined as in (7) [33].
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Vohm
r1 r2T I A
(7)
Where r1, r2 are ohmic resistance parameters [35], [36]. The Vcon is neglected in this modelling part since it has extremely low value compared to the Vact and Vohm. Hence, equation (4) can be rewritten as in (8). Vcell Vrev Vact Vohm
(8)
5.1. Simulation background Table 1 illustrates the relevant parameters including units which are used in mathematical model. Reference [35] and [36] are explained in the modelling part of the electrolysis cell and most of the data available in the table are taken from that research article. Table 1. Details of constants for mathematical modelling [35] Name of Constants
Symbols
Units
Value
Reversible Voltage Area of Electrode Faraday's Constant Number of Electrons The Coefficient for overvoltage on electrodes Coefficients for overvoltage on electrodes
𝑉𝑟𝑒𝑣 A F Z s 𝑡1 𝑡2 𝑡3
V 𝑐𝑚2 𝐶𝑚𝑜𝑙 ―1 V 𝐴 ―1𝑚2 𝐴 ―1𝑚2 0𝐶 𝐴 ―1𝑚2 0𝐶
1.229 0.25 96485 2 0.185 1.002 8.424 247.3
Parameters related to ohmic resistance of the electrolyte
𝑟1 𝑟2
𝛺𝑚2 𝛺𝑚2 0𝐶 ―1
8.05e-5 -2.5e--7
5.2. Mathematical model of the electrolysis cell A developed mathematical model in MATLAB/SIMULINK environment is illustrated in Figure 8. The operating temperature (T) and the area of the cell (A) are selected as inputs during this modelling study. The V-I characteristic and P-I characteristic of alkaline electrolyser obtained by mathematical model, are shown in Figure 9 and Figure 10 respectively. Furthermore, cell voltage and overvoltage characteristics are illustrated in Figure 11.
Figure 8. Mathematical model of the alkaline electrolysis cell
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Figure 9. V-I characteristic of the alkaline electrolysis cell
Figure 10. P-I characteristic of the alkaline electrolysis cell
Figure 11. Cell voltage and overvoltages of the alkaline electrolysis cell
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6. Experimental results and discussion In this research work, twelve different experiments have been conducted. At the very beginning, nine different experiments were done in order to enhance the performance of a single cell alkaline electrolyser by changing the properties of Membrane Electrode Assembly (MEA) and electrode distance of the cell. Thereafter, three experiments have been deployed to examine the performance of electrolysis stacks and the schematic view of the experimental setup for stack testing is illustrated in Figure 12. This paper is providing more detail and extended results of our work reported in Ref [37].
Figure 12. Schematic view of the experimental setup for stack testing 6.1. Experiment I: Commencement of the experiment The main objective of this experiment is to investigate working principle of the electrolysis process. For the experiment 30 wt% aqueous solutions of the KOH is deployed and, around 2.5 V electric current is supplied through the electrodes that are immersed in an electrolyte solution. The separating papers and nylon separator are placed between the anode and cathode to build up the membrane electrode assembly (MEA). Thereafter, all the parts of MEA are tied together to diminish the ohmic overvoltage and immersed inside the electrolysis solution to observe the electrochemical reactions. 6.2. Experiment II: Electrodes with electrocatalyst. In this experiment, an electrocatalyst is applied to electrodes in order to enhance the rate of electrochemical reaction which is taken place near the electrodes. A bench setup, which is illustrated in Figure 12, is used in this experiment as in Experiment I, and an electrocatalyst is coated to the electrodes of Experiment II. That is the only difference between Experiment I and Experiment II. The performance of Experiment I and Experiment II is compared with a mathematical model to investigate the performance of both experiments as shown in Figure 13. The results imply that the electrolysis cell with electrocatalyst exhibits better performance since the electrocatalyst is able to reduce the activation energy for the electrochemical reaction by increasing electrolysis cell efficiency.
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Figure 13. Performance comparison of Experiment I and II with the mathematical model. 6.3. Experiment III: Collecting the product gas using the first design. The product gas Collection is one of the important aspects of this research work. Thus, the MEA which is developed in Experiment II has been placed inside the cell frame to collect and measure the product gas flow rate. The electrolysis cell frame is designed using Solidworks as a design flat form. The flow rate of product gas was measured by using 100 ml measuring flask as illustrated in Figure 12. 6.4. Experiment IV: Enhancement of purity level of product gas through second design The purity level of product gas is a quite important parameter while investigating the performance of the cell. Thus, in this experiment, the purity level of product gas is enhanced by designing an intermediate storage tank on top of the intermediate base plate. The intermediate storage tank is able to condense water vapor before entering to the gas container by increasing the efficiency of the cell. Nevertheless, in Experiment III, the purity level of product gas is quite low, owing to the presence of water vapor. That problem can be mitigated by using the second design and the performance comparison of Experiment III and Experiment IV with the mathematical model is illustrated in Figure 14.
Figure 14. Performance comparison of Experiment III and IV with the mathematical model According to the performance comparison curve, Experiment IV exhibits comparatively a better performance owing to the small electrode distance that diminishes the ohmic loss of the cell by
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increasing efficiency. Moreover, the intermediate storage tank is able to enhance the performance of electrolysis cell. 6.5. Experiment V: Effect from the Thickness of Electrodes In this experiment, the thickness of the electrodes was enhanced by adding porous nickel plates to MEA. Same bench setup was deployed as in Experiment I, and the required data for efficiency and power calculations were obtained (Tables 2 and 3). The performance comparison was done through the V-I characteristic curve which was generated from a mathematical model. Table 2. Flow rate calculation data of Experiment V Produced Gases Cell Current (A) Voltage of Cell (V) Hydrogen (𝐻2(𝑔)) Oxygen (𝑂2(𝑔))
2.5 3.14 2.5 3.14
2.03 2.1 2.06 2.1
Time (s) 301 260 672 554
Table 3. Calculated parameters of Experiment V Flow rate calculation Power and efficiency calculation Variable Current of the cell (A) 𝑛𝐻2 (mol/min) 𝑛𝑜2 (mol/min)
Value
Variable
3.14 9.7632 x 10 ―4 4.8816 x 10
―4
Value
Voltage of cell (V) 𝑃𝑇𝑜𝑡𝑎𝑙 (W)
2.1 6.594
𝑅𝑂ℎ𝑚𝑖𝑐 (Ω)
0.207
𝑉𝐻2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛)
24.017
𝑃𝑂ℎ𝑚𝑖𝑐 (W)
2.0433
𝑉𝑂2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛)
12.008
𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (W)
4.5507
𝑉𝐻2(𝑔) (Exp)(𝑐𝑚 /𝑚𝑖𝑛)
23.077
𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (%)
69.01
𝑉𝑂2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
10.83
𝜂𝑒𝑛𝑒𝑟𝑔𝑦 (%)
70.5
3
6.6. Experiment VI: Effect of distance between electrodes The distance between electrodes of electrolysis cell is a critical fact affecting to the electrolysis performance, particularly power and efficiency. The performance of electrolysis cell could be increased by keeping an optimum distance between the anode and cathode while reducing the ohmic loss inside the cell. In order to investigate the effect of electrode distance for electrolysis process, three experiments were conducted while varying the distance between electrodes as 2.5mm, 1.9mm and 1.3 mm respectively. Figure 15 shows the bench setup of Experiment VI. The V-I characteristic curves of each experiment were drawn by considering experimental data and compared with mathematical model to examine the performance of the cell as shown in Figure 16. According to the observations of this experiment, a 1.3 mm thick electrode assembly exhibits better performance compared with other two experiments.
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Figure 15. Bench setup of Experiment VI
Figure 16. Comparison of the Experiments (Electrode gap 2.5mm, 1.9 mm, 1.3mm) with the mathematical model The results imply that the efficiency of electrolysis cell could be enhanced by decreasing electrode distance further since the electrode distance is directly proportional to the ohmic loss. Consequently, the ohmic loss can be diminished by reducing the electrode distance. Thus, the electrolysis cell with a 1.3mm thick electrode assembly has better performance compared to other experiments. 6.7. Experiment VII: Electrolysis cell with polymeric membrane separator In this experiment, Nafion 115 and Nafion 117 polymeric membranes were deployed as separating diaphragms respectively. According to the observed data, the Nafion 115 gave a good performance compared to the Nafion 117. Nevertheless, the performance of both polymeric membranes is much lower than when compared with other experiments. The performance comparison of both polymeric membranes with a mathematical model is shown in Figure 17.
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Figure 17. Performance comparison of both polymeric membranes with the mathematical model The performance comparison curve shows that the overvoltage losses of both polymeric membranes are extremely high compared with other experiments. The polymeric membranes are permeable to the water molecules and hydroxide ions. Nevertheless, the permeability resistance of membranes is higher than the electrode assembly of previous experiments. 6.8. Experiment VIII: Electrodes with porous stainless-steel plates To reduce the ohmic overvoltage, electrodes of the cell were replaced by porous stainless-steel plates and the relevant data was observed as in previous experiments by keeping the electrode gap as 1.3 mm. The observed data and calculated parameters are illustrated in Tables 4 and 5 respectively. Table 4. Flow rate calculation data of the Experiment VIII Produced Gases Cell Current (A) Voltage of Cell (V) Hydrogen (𝐻2(𝑔)) Oxygen (𝑂2(𝑔))
2.5 3.1 2.5 3.14
1.98 2.04 1.98 2.04
Time (s) 300 250 588 505
Table 5. Calculated parameters of the Experiment VIII Flow rate calculation Power and efficiency calculation Variable
Value
Current of the cell (A) 𝑛𝐻2 (mol/min)
9.7632 x 10
𝑛𝑜2 (mol/min)
Variable Voltage of cell (V) 𝑃𝑇𝑜𝑡𝑎𝑙 (W)
6.4056
4.8816 x 10 ―4
𝑅𝑂ℎ𝑚𝑖𝑐 (Ω)
0.194
23.017
𝑃𝑂ℎ𝑚𝑖𝑐 (W)
1.914
𝑉𝑂2(𝑔) (Theo)(𝑐𝑚 /𝑚𝑖𝑛)
12.01
𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (W)
4.4921
𝑉𝐻2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
24
𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (%)
70.13
𝑉𝑂2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
11.88
𝜂𝑒𝑛𝑒𝑟𝑔𝑦 (%)
72.5
𝑉𝐻2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛) 3
3.14
Value
―4
2.05
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6.9. Experiment IX: Electrolysis cell with fine nylon mesh In this experiment, one nylon mesh and two filter papers were adopted as separating diaphragms and, the same procedure was continued to observe the required data by keeping the gap between electrodes as 1.3 mm. Tables 6 and 7 shows the observed data and calculated parameters of Experiment IX. Table 6. Flow rate calculation data of the Experiment IX Produced Gases Cell Current (A) Voltage of Cell (V) Hydrogen (𝐻2(𝑔)) Oxygen (𝑂2(𝑔))
2.5 3.1 2.5 3.14
2 2.05 2 2.05
Time (s) 334 254 613 551
Table 7. Calculated parameters of the Experiment IX Flow rate calculation Power and efficiency calculation Variable
Value
Variable
Current of the cell (A) 𝑛𝐻2 (mol/min)
9.6388 x 10
𝑛𝑜2 (mol/min)
4.8194 x 10 ―4
𝑅𝑂ℎ𝑚𝑖𝑐 (Ω)
0.2
𝑉𝐻2(𝑔) (Theo)(𝑐𝑚 /𝑚𝑖𝑛)
23.71
𝑃𝑂ℎ𝑚𝑖𝑐 (W)
1.922
𝑉𝑂2(𝑔) (Theo)(𝑐𝑚3/𝑚𝑖𝑛)
11.86
𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (W)
4.433
𝑉𝐻2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
23.62
𝜂𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠 (%)
69.8
𝑉𝑂2(𝑔) (Exp)(𝑐𝑚3/𝑚𝑖𝑛)
10.89
𝜂𝑒𝑛𝑒𝑟𝑔𝑦 (%)
72.2
3
3.1 ―4
Voltage of cell (V) 𝑃𝑇𝑜𝑡𝑎𝑙 (W)
Value 2.05 6.355
Figure 18 is illustrated combinations of V-I characteristic curves of the Experiments V, VIII, and IX with the mathematical model as a comparison of particular experiments.
Figure 18. Comparison of the Experiments V, VIII, and IX with the mathematical model The performance comparison curve implies that the electrolysis cell in Experiment VIII has extremely better performance when compared with other experiments. In that experiment, we used
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porous electrodes by keeping electrode distance as 1.3 mm as in Experiment VI. The porous electrodes are able to reduce the internal losses of the electrolysis cell by increasing efficiency. 6.10. Experiment X: Implementation of monopolar electrolysis stack. The single cell alkaline electrolyser in the Experiment VIII is deployed while fabricating the monopolar electrolysis stack owing to the great performance compared with the other experiments. the stack consists of three single cells as illustrated in Figure 19.
Figure 19. Bench setup of Experiment X 6.11. Experiment XI: Implementation of bipolar electrolysis stack. Same as in Experiment X, the single cell alkaline electrolyser in the Experiment VIII is adopted while fabricating the bipolar electrolysis stack as well, owing to the great performance compared with the other experiments. the stack consists of three single cells as illustrated in Figure 20.
Figure 20. Bench setup of Experiment XI
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The performance comparison of monopolar and bipolar configuration of alkaline electrolysis stacks is one of the major objectives of this study. Hence, the V-I characteristic curve of both electrolysis stacks and mathematical model are drawn in the same axis to have a better idea about performance compression as shown in Figure 21.
Figure 21. Performance comparison of monopolar and bipolar configuration with the mathematical model According to the plot of performance comparison between monopolar and bipolar configuration of electrolysis stack, bipolar stack exhibits better performance than monopolar stack since the bipolar configuration is more compact and has a small ohmic loss compared to the monopolar equivalent. 6.12. Experiment XII: AC Impedance Test In this study, it is extremely important to obtain the impedance and the ohmic resistance of the cell. There are several methods available to analyze the AC impedance of the electrolysis cell such as the electrochemical impedance spectroscopy (EIS), the current sweep and the current interruption. Normally, the current sweep and the current interruption method cannot be used to determine the impedance of the cell during operation [38]. Consequently, the EIS is used to measure the ohmic resistance of the cell during the operation in this research study. The bench set up of Experiment XII is shown in Figure 22.
Figure 22. Bench setup for AC - impedance test
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The impedance is a complex quantity with a real and imaginary part. The real part of the impedance is directly proportional to the ohmic resistance of the electrolysis cell. The Nyquist plot is commonly used to display the impedance of the system. The AC impedance plots of monopolar and bipolar configuration of the electrolysis stack are illustrated in Figure 23 and Figure 24respectively.
Figure 23. AC - impedance plot of the monopolar electrolysis stack
Figure 24. AC - impedance plot of the bipolar electrolysis stack According to the impedance plot of the monopolar and bipolar electrolysis stack, the ohmic resistance of the stack is observed as 0.714 Ω and 0.621 Ω respectively. The bench set up of Experiment XII is shown in Figure 24. In this research study, a comparison of bipolar and monopolar electrolysis stacks was presented in detail. First, two different single cell electrolysers were developed and its performance was examined to select the best performing cell for the stack design. According to the observations, the second design of single cell electrolyser was selected for the stack design because of its performance. Consequently, electrolysis efficiencies of the first and the second designs were 67.9% and 69.5% respectively. Prior to that, the purity level of product gas was also considered while selecting the best performing cell. Especially, in the second design, the purity level of product gas was enhanced by an intermediate storage tank that was able to condense water vapor before entering to the gas container. The gas mixing issue of this research work was eliminated by introducing polymeric membrane and filter papers for MEA which enhance the purity level of product gas by a considerable amount. The purity level of product gas was enhanced in Ref [19], by decoupling anode and cathode through nickel hydroxide as a redox mediator to overcomes the gas-mixing issue. Thereafter, the electrolysis efficiency of the second design was enhanced further from 69.5% to 72.5% (Experiment VIII) through several experiments by changing materials and electrode distance, before using it for stack design. Thus, the bipolar and monopolar configuration of alkaline electrolysis stack (3-cell stack) was developed by using the electrolysis cell which was developed under Experiment VIII. Consequently, the electrolysis efficiency of the bipolar
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and monopolar stack was calculated as 67.9% and 69.5% respectively. The results imply that the designed bipolar stack has a better performance in terms of efficiency, power and flow rates than the monopolar equivalent. The same procedure was adopted in Ref [39], for developing and testing of a highly efficient proton exchange membrane (PEM) electrolyzer stack. In that research study, the author emphasized that future studies need to be done to reduce manufacturing cost by employing cheaper materials or coating. In this research work, as a cell material, we employed high quality, durable dense plastic and 3D printing technology to reduce the cost associated with post machining process. The comparable bipolar and monopolar connection of capacitive deionization stack in NaCl treatment was discussed in previous studies [40]. In that study, both bipolar and monopolar configuration of electrolysis stack was designed, and they were compared for the influence of the voltage distribution depending on the cell structure. The bipolar circuit structure was more favorable to adsorb the ions electrostatically to the electrode surface although there are voltage balancing problems between each cell of the stack. The bipolar and unipolar configuration of alkaline electrolysis stack, solid polymer electrolyte, and vapor-phase technologies are compared in Ref [41]. It is concluded that alkaline electrolysis at temperatures below 150°C will be economically preferred over the next decade, while vapor-phase electrolysis is a development of significant but longer-term interest. Thus, the manufacturing cost of the alkaline electrolysis stack was comparatively lower than other existing technology and the bipolar stack exhibit better performance when compared with the monopolar stack. Nevertheless, optimization of the stack performance by using different materials for electrodes requires further research. 7. Conclusion In this research, we have enhanced the performance of the electrolysis process by decreasing the distance between the anode and cathode, while changing the properties of the Membrane Electrode Assembly (MEA). The main aim of this research was to design and fabricating of mono-polar and bi-polar configuration of alkaline electrolysis stacks to compare their performance through 3-D modelling and experimental fabrication. The first four experiments have been used as benchmark tests to determine the best single cell configuration which was able to provide adequate product gas flow rate with high purity level. Thus, two designs of single cell alkaline electrolysis cells have been deployed, and the second design has been selected as the best performing cell after considering their performance separately. The different experiments were carried out to improve the performance of the selected cell and Experiment VIII has been selected as the best performing cell after considering the performance comparison graph with the mathematical model. During that experiment, the distance between porous electrodes was set to 1.3 mm to reduce the ohmic overvoltage inside the cell, by increasing the efficiency and power. According to the observations of that experiment, the practical values of hydrogen gas flow rate, oxygen gas flow rate and electrolysis efficiency have been calculated as 24 cm3/min, 11.88 cm3/min and 70.13% respectively. The bi-polar and mono-polar configurations of electrolysis stack have been fabricated by using the electrolysis cell, which has been developed in Experiment VIII, and the rest of the experiments have been implemented to investigate the performance of fabricated electrolysis stacks. The observation implies that the bi-polar electrolysis stack exhibited better performance than the mono-polar electrolysis stack. Furthermore, the hydrogen gas flow rate, the oxygen gas flow rate and the electrolysis efficiency have been calculated for bi-polar configuration, as 71.43 cm3/min, 35.96 cm3/min and 69.88 % respectively. Moreover, the bi-polar stack is more compact than the mono-polar stack and the AC impedance tests have been implemented to obtain the exact values of ohmic loss inside both stacks. Nafion has been deployed as a catalyst layer to enhance the rate of HER and OER. Particularly, the Nafion is an acid and reacts with the alkaline solution by reducing the lifetime of the cell. Nevertheless, the life expectancy of the cell was more than adequate for experimental purposes. In the future, we intend to develop a zero-gap electrolysis cell with different catalyst layers to reduce the ohmic loss further and, an optical sensing technology to be integrated for observing the optimum distance between the electrodes quite accurately. It is obvious that all the observations of this research study indicate that the designed bi-polar stack has a better performance in terms of efficiency, power and flow rates than the mono-polar equivalent.
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Journal Pre-proof Author Contribution Statements I, Y. Sanath K. De Silva attached to the Faculty of Engineering and Science, University of Agder, Norway, as an MSc student. Presented paper is a part of my master study. Prof. Peter Hugh Middleton and Prof. Mohan Lal Kolhe were my supervisors. The presented idea developed by Prof. Peter Hugh Middleton and Prof. Mohan Lal Kolhe developed the theory and performed the computations. All authors discussed the results and contributed to the final manuscript. Y. Sanath K. De Silva carried out the experiment and wrote the manuscript with support from Prof. Peter Hugh Middleton and Prof. Mohan Lal Kolhe. All authors contributed to the final version of the manuscript. Prof. Peter Hugh Middleton supervised the project. All authors provided critical feedback and helped shape the research, analysis and manuscript and discussed the results and commented on the manuscript.
Journal Pre-proof Declaration of Interest Statements
The University of Agder, Norway has equity ownership in the paper of “Performance Comparison of Mono-polar and Bi-polar Configurations of Alkaline Electrolysis Stack through 3-D Modelling and Experimental Fabrication”. The University of Agder, at Norway may benefit from this interest, if the research article is successfully published in this journal. The terms of this arrangement have been reviewed and approved by the University of Agder, Norway in accordance with its policy on objectivity in research.
Journal Pre-proof Highlights
Alkaline electrolysis configuration (i.e. mono-polar and bi-polar) modelling for performance analysis.
Membrane electrode assembly (MEA) configuration effect on electrolysis performance.
Improvement of electrolysis performance with bi-polar stack compared to mono-polar equivalent.